VLT FORS2 comparative transmission spectroscopy: Detection of Na in the atmosphere of WASP-39b from the ground

TL;DR

This study demonstrates ground-based optical transmission spectroscopy of exoplanet WASP-39b using VLT FORS2, detecting sodium and potassium features and confirming a clear atmosphere consistent with HST data.

Contribution

It shows that the upgraded FORS2 instrument can achieve HST-quality transmission spectra from the ground, enabling detailed atmospheric characterization.

Findings

Detection of sodium absorption at 3.2-sigma significance

Evidence for potassium in the atmosphere

Ground-based spectra consistent with HST results

Abstract

We present transmission spectroscopy of the warm Saturn-mass exoplanet WASP-39b made with the Very Large Telescope (VLT) FOcal Reducer and Spectrograph (FORS2) across the wavelength range 411-810nm. The transit depth is measured with a typical precision of 240 parts per million (ppm) in wavelength bins of 10nm on a V = 12.1 magnitude star. We detect the sodium absorption feature (3.2-sigma) and find evidence for potassium. The ground-based transmission spectrum is consistent with Hubble Space Telescope (HST) optical spectroscopy, strengthening the interpretation of WASP-39b having a largely clear atmosphere. Our results demonstrate the great potential of the recently upgraded FORS2 spectrograph for optical transmission spectroscopy, obtaining HST-quality light curves from the ground.